Pepsin is used to break down protein in the stomach by hydrolyzing some peptide bonds. Pepsin can operate best at a pH of 2 (same pH as the stomach). Adding HCl to the solution will bring the solution's pH closer to this optimal pH.

Lysosomes are responsible for the degradation of macromolecules, and typically have an internal pH of 5. They contain acid hydrolases: enzymes that function optimally in an acidic environment.

Kinases are protein enzymes that add phosphate groups to targets. These targets can be diverse, however, and are not always translation factors.

The diagram in the passage shows the kinase PERK, which must phosphorylate its substrate, acts on eIF2. Based on its kinase nature and the diagram, phosphorylation of eIF2 is the most likely answer that would lead to propagation of the signal shown.

While enzymes do not change the amount of product formed in a reaction (no change to K_eq) they do speed up the rate of reaction. It is also true that under certain conditions pH and/or heat can denature an enzyme.

During a reaction, an enzyme does not get used up and is regenerated; enzymes are a type catalyst. Essentially, the enzyme is both a reactant and a product of the reaction it catalyzes.

Keep in mind that enzymes are proteins. They will increase in efficiency as temperature increases, but eventually too much heat will start to denature the protein. As a result, the graph will climb to maximum effeciency at a specific temperature. After that peak, it will decrease due to the denaturing of the enzyme.

Very low temperatures result in very low functionality. Mid-range temperatures result in maximum functionality. Very high temperatures result in very low functionality. As a result, the graph will be shaped like a bell-curve.

As substrate concentration is increased, the reaction rate will increase accordingly; however, let's think about the extreme case where there is an extremely large amount of substrate. Eventually, every binding site of every molecule of enzyme will be filled. Substrate molecules will have to wait in order to be catalyzed by the enzyme. When this happens, we say that the enzyme is saturated. At this point, the graph will begin to level off and look like a horizontal line.

In summary, the graph will rise quickly in the beginning, but will eventually level off as substrate concentration becomes excessive compared to the available enzyme in solution.

The efficiency of an enzyme is dependent on the pH (as well as other features) of the environment in which it acts. The pancreatic digestive enzymes are typically secreted into the small intestine, which has a pH of about 6. As a result, the acidic pH of the stomach (about 2) would significantly reduce the efficiency of the pancreatic enzymes.

Remember that, though the stomach contents is highly acidic, it is neutralized in the duodenum before continuing through the small intestine, thus allowing these enzymes to function.

Zymogen is the correct term for the inactive precursor of an enzyme. Zymogens are cleaved by other enzymes in order to become active. The zymogen form can help prevent improper action of the enzyme in different regions of the body. For example, trypsinogen is a zymogen released from the pancreas. It is transported to the small intestine before become active trypsin to prevent the trypsin from accidentally digesting and damaging the pancreatic cells.

Apoenzymes refer to enzymes without cofactors, while holoenzymes are enzymes bound to their cofactors. Inhibitors bind to enzymes to block their activity.

Enzymes are pH and temperature sensitive., and only function in optimal ranges of these conditions. Certain enzymes will only function in acidic environments, while others require basic conditions.

The overall free energy and enthalpy of the reaction, activation energy, and enzyme concentration do not have a bearing on enzymatic activity.